(1) Field of the Invention
The present invention relates to a color picture tube, in particular to a color picture tube in which flatness of an image display portion and visibility is improved.
(2) Description of Related Art
In general, a color picture tube that is employed in a TV Braun tube, computer display or the like has a vacuum envelope consisting of a glass panel and a glass funnel of funnel shape. On an internal surface of the effective area that is an image display portion of the panel, a phosphor screen comprising phosphor layers of three colors, blue (B), green (G) and red (R), is formed. Electron beams emitted from electron guns that are disposed inside the neck of the funnel are deflected by a magnetic field generated by a deflection yoke disposed outside the funnel to scan the phosphor screen in a horizontal and vertical direction through a shadow mask, resulting in a display of color images.
The panel of the color picture tube comprises an effective area whose plane shape is essentially rectangular and a sidewall disposed in the surrounding area thereof. So as to obtain strength capable of supporting the weight of atmosphere applied on the vacuum envelope, the internal and external surfaces of the panel are formed with different curvatures, and the thickness thereof is formed to become thicker from the center portion of the effective area towards a periphery portion thereof.
In recent years, in order to improve visibility of images, the external surface of the effective area of the panel is being made flat (flattening). In order to improve the visibility, it is also necessary for the internal surface of the effective area to be flat. Nevertheless, in order to secure strength to support the curved surface of a shadow mask disposed inside the panel and strength of the vacuum envelope supporting atmospheric pressure, it had been extremely difficult to make the internal surface of the effective area of the panel flat to the level comparable with the external surface. In addition, from a viewpoint of the characteristics of a color picture tube, it was difficult to make the internal surface of the effective area flat.
Further, a phosphor screen is formed in contact with the internal surface of the effective area of the panel. The substantial image display portion is, when not taking into account the refractive index of the glass panel, a curved face that is the same as the internal surface of the effective area. Accordingly, so far there was a problem that images of high flatness could not be obtained without making the internal surface flat.
An object of the present invention is that in a color picture tube where the effective area of a panel is substantially rectangular, the internal surface of the effective area thereof is formed in a curved face having improved visibility to attain a better sense of flatness of images. At the same time, the strength that supports a curved surface of a shadow mask is secured and deterioration of color purity due to local thermal expansion of the shadow mask is prevented from occurring.
The present invention is a color picture tube, comprising a transparent panel of which an external surface is flat, and an internal surface is a curved face and a plane shape with a substantially rectangular effective area, and a phosphor screen disposed on the internal surface of the effective area of the panel, wherein RH greater than RV and RSxe2x89xa7RV, where RH is an average radius of curvature in major axis direction and denotes a radius of a curved line (an arc) that goes through a center of the internal surface of the effective area and ends on a major axis, RV is an average radius of curvature in minor axis direction and denotes a radius of an arc that goes through the center and ends of a minor axis on the internal surface of the effective area, and RS is an average radius of curvature in short side direction and denotes a radius of an arc that goes through an end of the major axis and ends of diagonal axes on the internal surface of the effective area.
Further, in the color picture tube according to the present invention, the internal surface of the effective area can be formed with a curved face that satisfies the following expressions (1), (2) and (3).
RH greater than RVxe2x80x83xe2x80x83(1)
RSxe2x89xa7RVxe2x80x83xe2x80x83(2)
RLxe2x89xa7RHxe2x80x83xe2x80x83(3)
Here RL is an average radius of curvature in long side direction and denotes a radius of an arc that goes through an end of the minor axis and ends of the diagonal axes of the internal surface of the effective area.
In addition, the internal surface of the effective area can be formed with a curved face in which long sides that go through the end on the minor axis and the ends on the diagonal axes exist approximately on one plane.
Further, in a color picture tube in which the internal surface of the effective area of the panel satisfies all the aforementioned expressions (1), (2) and (3),
wherein the internal surface of the effective area can be formed with a curved face that further satisfies the following expressions (4) and (5).
2 less than RH/RV less than 15xe2x80x83xe2x80x83(4)
1 less than RH/RD less than 10xe2x80x83xe2x80x83(5)
Incidentally, RD is an average radius of curvature in diagonal axis direction and denotes a radius of an arc that goes through the center and the ends of the diagonal axes of the internal surface of the effective area.
Further, in such a color picture tube, the internal surface of the effective area can be formed with a curved face that satisfies the following expression (6).
xe2x80x831 less than RS/RV less than 5xe2x80x83xe2x80x83(6)
Still further, the internal surface of the effective area can be formed in a curved face that satisfies the following expression (7).
3 less than RL/RHxe2x80x83xe2x80x83(7)